World No. 1 men's golfer Jordan Spieth fired a "stress-free" four-under 67 to hover near the top of the leaderboard at the opening day of the SMBC Singapore Open.

Followed by a crowd of over 600 under the sweltering heat at the Sentosa Golf Club, the American sensation found 10 out of 14 fairways.

But he needed 31 putts to finish his round, some way off the sizzling form on the greens that sparked his US Open and Masters victories last year.

On his bogey-free outing, the 22-year-old said: "It was stress-free driving the ball but there was no real opportunity after.

"It could be the heat and humidity, or it could be the course, which has met my expectations on how tough it plays.

"It's still a good start, just the first round. I'm still adjusting to the time zone."

With its tight fairways and zippy greens, Serapong's notoriously tricky setting proved to be a stern challenge at times for the man who has held golf's top ranking for 12 consecutive weeks.

Spieth swung his club skywards in frustration after a skewed approach shot landed 15 feet away from the pin on the par-five 18th. His trusty putter, however, helped to salvage an unlikely birdie.

South African Keith Horne currently leads the pack after carding an impressive 66, while Quincy Quek is Singapore's top performer on 68.

The US$1 million (S$1.44 million) event is co-sanctioned by the Asian and Japanese Tours.